Corrosion resistant steels having improved weldability

ABSTRACT

Corrosion resistant steels having improved weldability particularly sea-water resistant steels comprising 0.001 - 0.25 percent of carbon, 0.7 - 1.1 percent of silicon, 0.3 - 2.0 percent of manganese, 0.5 - 2.0 percent of chromium, 0.1 - 1.5 percent of molybdenum. 0.1 - 0.29 percent of copper, less than 0.04 percent of phosphorus, two or more elements selected from the group consisting of arsenic, tin, beryllium, bismuth, lead, germanium, antimony, selenium and tellurium in an amount of 0.02 - 0.20 percent each, the balance being iron and unavoidable impurities.

ilnite totes atent [1 1 Nishi et al.

CORROSION RESISTANT STElElLS HAVING IMPROVED WELDABIILITY Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: July 5, 1972 Appl. No.: 269,084

Related US. Application Data Continuation-impart of Ser. Nos. 2,695, Jan. 14, 1970, Pat. No. 3,733,195, and Ser. No. 237,215, March 22, 1972, abandoned, which is a division of Ser. No. 2,695.

11.8. C1. 75/125, 75/126 M, 75/126 N Int. Cl. C226 39/54, C22c 39/20 Field of Search 75/126 L, 126 M, 126 N,

References Cited UNITED STATES PATENTS 6/1935 Strauss 75/125 Dec. 10, 1974 2,102,283 12/1937 Saklatwalla 75/125 3,070,438 12/1969 Kenneford 75/125 3,110,798 11/1963 Keay 75/125 3,368,886 2/1968 Shimizu 75/125 3,403,060 9/1968 Ito 75/125 3,443,934 5/1969 Kubota 75/128 P 3,679,400 7/1972 Nachtman... 75/126 M 3,684,493 8/1972 Kubota 75/125 Primary Examiner-Hyland Bizot Attorney, Agent, or FirmWenderoth, Lind & Ponack ABSTRACT Corrosion resistant steels having improved weldability particularly sea-water resistant steels comprising 0.001 0.25 percent of carbon, 07 1.1 percent of silicon, 0.3 2.0 percent of manganese, 0.5 2.0 percent of chromium, 0.1 1.5 percent of molybdenum. 0.1 0.29 percent of copper, less than 0.04 percent of phosphorus, two or more elements selected from the group consisting of arsenic, tin, beryllium, bismuth, lead, germanium, antimony, selenium and tellurium in an amount of 0.02 0.20 percent each, the balance being iron and unavoidable impurities.

1 Claim, No Drawings CORROSTON RESTSTANT STEEILS ll-llAVllNG IMPROVED WlElLDABlLllTY This is a continuation-in-part of copending applications Ser. Nos. 2,695, filed Jan. 14, 1970, now U.S. Pat.

only weather-resistance alone cannot be used as seawater resistant steel materials.

Presently. cathodic protection by using galvanic anode drainage using zinc, aluminum, magnesium etc.

those ofordinary weather-resistant steel materials (particularly corrosion by industrial atmospheres) and is characterized by severe local corrosion and pit corrosion. This is the very reason why steel materials having No. 3,733,195 and Ser. No. 237,215, filed Mar. 22, 5 and corrosion inhibitors have been proposed for pre- 1972, now abandoned, said application Ser. No. venting such types of corrosion as above. But, all of 237,215 being a division of said application Ser. No. these methods are only temporarily effective and re- 2,695. quire very large cost and labor for maintenance.

Further, steels as shown in Table l have been pro- BACKGROUND OF INVENTION 0 posed for such marine applications and are improved in sea-water resistance as well as in mechanical proper- Conventional sea-water resistant steels have the folties by addition of phosphur, copper, chromium and lowing corrosion problems. aluminum.

" Table l C Si Mn P S Cu Cr others 1') (7d 0.6 0.08 Ni: 0.22 0.10 -09 -0.15 do. 0.5 0.40 0.65

0.6 0.5 A1: 0.18 0.50 0.6 do. 0.03 -1.5 0.5 1.5

3.9 A1: 0.13 do. 0.5 0.03 0.025 -4.3 0.7 1.1

1. In case of marine structures such as base piles and The above conventional sea-water resistant steels are buoys which are exposed and splashed with sea-water improved in sea-water resistance chiefly by the addithe corrosion of portions splashed with sea-water is as 30 tion of phosphorus and/or copper, but these additives high as 3 to 7 times the corrosion of other portions present a critical defect that cracks at welded portions (portions that are in the sea-water or above the sea), are remarkably increased in welded structures such as the corroded surface conditions are characterized by submarine pipe lines and buoys. severe local corrosion and pit corrosion, and the corro- Further, many trials have been proposed for improvsion products (rust) of the splashed portions have reing sea-water resistance of steels by the addition of spemarkable tendency of sticking as compared with the cial elements. For example, Hudson teaches in Journal other portions. of Iron and Steel Institute (H51), 1955, July, vol. 180,

2. In case of applications such as marine steel piping Pl'f- 284 that Chromium, phsophorus, silicon, aluand floats which are immersed always in the sea-water, 4O r molybdenum mck el i beryll'um are useful sevrre corrosion takes place particularly on portions of whlle arsemc blsmulh lead tungsten the structure which are 1 3m below the sea surface. niobium and Vanadlum not This is due to the physiological action of sulphate- However, Hudson's fmdlngs are llmlted to results P- reducing bacteria which reduces sulphate ions in the mined from Studies on Corrosion sea'water and sea-water into sulphur and corrode the steel remark- P Water, and does not teach of Corrosion Faused y ably. This action is greater in contaminated zones and repeated Splashing of the Sea-water corroslon under warm districts f the a condition where both crude oil and sea-water are 3. In case of applications such as oil tankers, oil pipe present type and mechanism of corroslon under lines which are exposed alternately or simultaneously such confmlon are qute dlfferem from those m with the sea-water and oils. water or m plam water The corrosion is a complicated combination of DESCRIPTION OF INVENTION fhole'surface corrosion and loFal corroslol" f The present invention relates to corrosion resistance rate of the lafterfype g extraorfjmanly steels, particularly sea-water resistant steels, having imgenerali F' m alone y Small proved weldability, useful for applications such as ship and for acceleration of corrosion, an electrolync aquehulls, buoys landing piers, base pines, dolphins (marine l soluuoni Such and P131" water oil drilling platforms) which are exposed to splashing of tamed 1n the crude 011 plays an 1mportant role. Further Seawater, Such as marine or submarine Steel pipes and acceleration of corros1on is effected by presence of floats which are always immersed in Seawater and Sludge and bactena m the crude t such as parts of ship hull and oil pipe lines which are In such marine applications as above, the corrosion exposed alternately or simultaneously to oil, sea-water of steel materials is due to the physiological action of and plain water.

Sulphate-reducing bacteria during the reduction of sul- The basic composition of the present invention steels phate ions contained in the sea-water into sulphur, and comprises 0.001 0.25 per cent by weight of carbon, thus the corrosion mechanism is quite different from 0.7 1.1 per cent by weight of-silicon, 0.3 2.0 per cent by weight of manganese, 0.5 2.0 per cent by weight of chromium, 0.1 1.5 per cent by weight of molybdenum, 0.1 0.29 per cent by weight of copper less than 0.04 per cent by weight of phosphorus, two or more elements selected from the group consisting of arsenic, tin, beryllium, bismuth, lead, germanium, antimony, selenium and tellurium in an amount of 0.02 0.20 per cent by weight each, the balance being iron and unavoidable impurities.

In the above preferred type of composition, the reasons for the limitations of each of the elements are as follows.

Silicon and manganese are well known as elements necessary for deoxidation in steel production, and chromium and copper are well known as elements necessary for improving strength and weather-resistance of steels.

However, in the present invention, when these elements are present in suitable amounts, a remarkably improved corrosion resistance, which cannot be expected from the addition of only one of these elements,

can be obtained.

For example, when both silicon and chromium are present in the steel, corrosion in contaminated seawater is remarkably reduced. ln other words, the resistance against general corrosion increases, as the chromium content increases. On the other hand, the resistance against pitting corrosion decreases as the chromium content increases. Therefore, the possibility of improving both the resistance against general corrosion and pitting corrosion was studied by the addition of other elements to silicon and chromium. From experimental results, it was found that the addition of molybdenum is remarkably effective for reducing pitting corrosion and general corrosion.

Particularly, an increased content of silicon is effective for forming a silicate film on the surface of the corrosion product (metal oxides) and thus effective for preventing corrosion action of the sulfate-reducing bacteria in the sea-water.

The present inventive steel having a composition as above shows an excellent sea-water resistance in a contaminated sea zone or in a sea-water splashing zone, and the proportional range of each element is defined as above because a steel having a composition in the above range is useful from the points of corrosion resistance, manufacturing, mechanical properties and weldability.

Namely, carbon is an element necessary for required strength of the present inventive steel. However, more than 0.25 percent of carbon causes embrittlement of the steel and lowers weldability and sea-water resistance, while less than 0.001 percent of carbon lowers steel strength and gives poor economy and productivity caused by a longer refining time. Silicon is necessary for deoxidation in steel production, and 0.7 l.l percent of silicon in combination with chromium and manganese improves sea-water resistance. In the amounts of silicon less than 0.7 percent, these effects are not substantial. On the other hand, the addition of silicon over 1.1 percent is disadvantageous for weldability. Manganese is useful as a deoxidizer and desulfurizer in steel production and is useful for improving steel strength. ln the present inventive steel more than 0.3 percent of manganese is necessary for the required strength, but too much manganese will cause embrittlement of the steel and is thus limited to 2.0 percent. Molybdenum coexistent with silicon and chromium is effective for reducing the pitting corrosion in marine atmosphere. This effect can not be expected with less than 0.1 percent of molybdenum and more than 1.5

percent of molybdenum will cause embrittlement. Copper is useful for improving sea-water resistance. This effect can not be expected with less than 0.1 percent of copper, and more than 0.29 percent of copper will cause embrittlement and lowers weldability without any remarkable improvement in sea-water resistance. Therefore, the content of copper is limited to 0.1 0.29 percent. An excess amount of phosphorus embrittles the steel, and severely deteriorates joint portions of welded structures such as sub-marine pipe lines and buoys. Thus, the upper limit of phosphorus is 0.04 percent. Sulphur, an unavoidable impurity, severely lower sea-water resistance and weldability, and should be maintained as low as possible. A permissible upper limit of sulphur is 0.035 percent.

A second preferred composition according to the present invention further comprises in addition to the first type of preferred composition one or more elements selected from the group consisting of zirconium, vanadium and tungsten, in an amount of 0.01 0.50 percent by weight with respect to zirconium and vanadium alone or combination; and 0.1 1.5 percent by weight with respect to tungsten alone or in combination.

For the types of preferred composition according to the present invention, addition to the basic composition (chromium, silicon, manganese, copper, carbon, phosphorus and molybdenum) of the second group of alloying elements (zirconium, vanadium and tungsten) or addition of the first group of alloying elements (arsenic, antimony, tin, lead, selenium, beryllium, bismuth, tellurium and germanium) is usefal to some degree for the desired corrosion resistance, but the addition of these two groups of alloying elements in combination gives remarkably improved and unique corrosion resistance which cannot be expected from the addition of either of the two groups alone.

The elements of the basic composition and second group by themselves are effective to reduce hydrogen embrittlement, prevent intergranular corrosion and improve corrosion fatigue, but their effects are remarkably enhanced when added in combination of the elements of the first group. This is due to the fact that the elements of the first group lower the activity coefficient of nitrides or carbides of the basic composition and second group elements to convert them into a useful state and give a strong protection against severe corrosion attach by the co-presence of crude oil and sea-water, as well as remarkably reduce corrosion on portions splashed with sea-water and corrosion by bacteria in a contaminated sea zone.

The addition of the first group elements by themselves is effective to reduce corrosion due to their toxicity to bacteria. Their effects are increased two-fold when added in combination of the second group elements, and further a remarkably improved resistance against local corrosion is given by a unique effect which reduces the ability of corrosion products to form a cathode.

Particularly, chromiun and silicon and useful for the above combined effect. Silicon and manganese have a specific ability to remarkably reduce corrosion on portions splashed with sea-water or corrosion under the condition where crude oil and sea-water are present, particularly when added in combination with copper.

Therefore, in view of economy, productivity, and practical utility, silicon, chromium, manganese, copper and molybdenum are indispensible elements in the present invention. As for carbon and phosphorus, they are necessary for desired workability and weldability.

The reason for addition of two or more elements of a converter or electric furnace, cast into i rolled.

A. shows general corrosion rates after rust removal determined on the specimens of4 mm thickness ngots and hot the first alloying group isthat although add1t1on ofarse- 5 X 50 mm width X 100 mm Hength, polished on a" me alone, for example, gives good corrosion resistance, surfaces with NO. 320 emergy and immersed element fa be a large amount izontally for one month alternately in a tank vlew of weldablhty and mechamcal P p on the filled with crude oil and then in a tank filled with other hand, with the addition of a decreased amount of Seawaten h corrosion rates Shown i A arsemc together h other element such as resents the ratio of the weight loss due to general proved Can be Obtained Sacrifice of corrosion of the present inventive Steel to that of corrosion resistance. Thus, taking one consideration h di carben Steel.

With another, it is advantageous to add or more Of B. shows the highest corrosion rates observed in the the first group of alloying elements. bov m nti ned specimens.

The Second W Or Steel Ofthe Present invention The highest corrosion rates in (B) represents the ther Comprises, in addition to the above composltloni ratio of the pit depth due to pitting corrosion of the one or more elements selected from the e ond gr p present inventive steel to that of the ordinary carbon of alloying elements in an amount of 0.01 0.50 pert l,

Cent by eight with re p t o a V, and 0.1 1.5 As seen from the Table, the present inventive steels percent by weight with respect to W, to give further imshow excellent corrosion resistance.

Table 2 Chemical Com osition Wei ht% No. of C Si Mn S Cu Cr Mo As Sn Specimens Ordinary carbon steel Inventive steel Sea water Sb Se Te Be Bi Pb Ge Corrosion Rate A 8 Ordinary carbon steel 1 100 100 lnventive steel 1 0.03 59.8 49.1 2 0.02 0.01 w;0.15 57.6 47.8 3 0.02 0.03 59.7 47.5 4 0.01 0.01 55.7 47.1 5 v;0.03 56.8 48.7 6 0.01 0.01 43.5 41.1 7 Zr;0.05 41.6 38.6 8 0.02 0.01 48.7 41.5 9 0.01 0.02 39.2 30.6

proved corrosion resistance and mechanical properties. What is claimed is: The lower limits of the above additional elements are 1. A corrosion resistant steel having improved weldshown as limits for tangible resistance against corroability conslstmg of 0.001 0.25 percent by weight of sion, while the upper limits are shown as limits beyond carbon, 0.70 l.l percent by weight of SlllCOIl, 0.3 which detrimental effect begin to appear on steel- 2.0 percent by weight of manganese, 0.5 2.0 percent making, or mechanical properties, weldability and by weight of chromium, 0.1 1.5 percent by weight of other practical properties. molybdenum, 0.1 0.29 percent by weight of copper,

The present inventive steels having a composition as 1655 than 0804 Percent y Weight of Ph05ph0ru5, and d fi above may b used h ll d two or more elements selected from. the group consistnealed" or as tempered. ing of arsenic, tin, beryllium, bismuth, lead, germa- The present invention will be better understood from nium, n m ny. Selenium d el urium in an amount the following example. of 0.02 0.20 percent by weight each, the balance being iron and unavoidable impuritien. Example Table l shows results of corrosion tests on the present inventive steels and conventional steels produced in 

1. A CORROSION RESISTANT STEEL HAVING IMPROVED WELDABILITY CONSISTING OF 0.001-0.25 PERCENT BY WEIGHT OF CARBON, 0.70 -1.1 PERCENT BY WEIGHT OF SILICON, 0.3-2.0 PERCENT BY WEIGHT OF MANGANESE, 0.5-2.0 PERCENT BY WEIGHT OF CHROMINUM, 0.1 -1.5 PERCENG BY WEIGHT OF MOLYBDENUM, 0.1-0.29 PERCENT BY WEIGHT OF COPPER, LESS THAN 0.04 PERCENT BY WEIGHT OF PHOSPHORUS, AND TWO OR MORE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF ARSENIC, TIN, BERYLLIUM, BISMUTH, LEAD, GERMANIUM, ANTIMONY, SELENIUM AND TELLURIUM IN AN AMOUNT OF 0.02-0.20 PERCENT BY WEIGHT EACH, THE BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES. 